From what I know, it is best to simply enjoy the various activities the game has to offer while racking up coins on the way. I normally do a few Normal/Hard Garden Ops and end up with 200,000 and over coins without noticing. I usually do some Mixed Multiplayer and eventually get the same results without the game feeling like a grind or a chore.
Long story short, this game is not too stingy with coin distribution. Checking the Quest Board and accepting easy quests (usually meeting the criteria that you normally meet anyway such as assist, etc) will grant you easy coins.
Also, there is promoting a character that gives a good 20,000 coins each time they go up 10 levels. Try and play as many characters as possible to get these bonuses plus it even raises your Player Rank!
